53 o THE POPULAR SCIENCE MONTHLY. 



and within a few years the multitude of isolated observations have 

 been collected, classified, and made available. The importance of this 

 undertaking will be more appreciated in the future than it has been in 

 the immediate past. In all cases of chemical change, energy in the 

 form of heat is either developed or absorbed, and the amount is as 

 definite in a given reaction as are the weights of the substances con- 

 cerned ; hence, measurement of the quantity of heat set free or ab- 

 sorbed in chemical reactions often enables the chemist to determine 

 the true nature of the change. For example, the exact condition of 

 certain bodies in solution can only be conjectured from certain physi- 

 cal characters, few and ill-defined ; but by thermic methods of inves- 

 tigation the bodies formed can be accurately ascertained. This is ac- 

 complished by reference to the law of maximum work : " In any 

 reaction, those bodies, the formation of which gives rise to the great- 

 est development of heat, are formed in preference to others." Thus 

 the thermometer alone in skillful hands determines the a priori ne- 

 cessity or impossibility of a reaction. 



Berthelot, in Paris, and Thorn sen, in Copenhagen, have pursued 

 the subject of thermo-chemistry with indefatigable zeal, and their 

 published results form monuments of exhaustive research. " By the 

 labors chiefly of these two men, we now know the thermal values cor- 

 responding to many thousands of chemical reactions. We have learned 

 that the energies of a reaction which can be brought about in two 

 methods, either in the dry way or by solution, differ in the two cases ; 

 that salts in solution are in a partial state of decomposition ; that the 

 attraction of a polybasic acid radical is not the same for the successive 

 portions of base added, and that the behavior of a monobasic acid in 

 solution differs essentially from that of a dibasic or tribasic acid. We 

 also know that the total energy involved in any reaction is largely in- 

 fluenced by the surrounding conditions of temperature, pressure, and 

 volume." 



The interesting border-line between chemistry and physics is an 

 increasing subject of research on the part of both the chemist and the 

 physicist. The periodic press chronicles profound studies of the rela- 

 tions between chemical constitution and the phenomena of diffusion, 

 of capillarity, of dialysis, of dissociation, and of the law of isomorphism. 

 We read investigations on the value of the theory of atomicity, and 

 on the nature of nascent action. Researches in the domain of electro- 

 chemistry, especially in connection with the various forms of storage- 

 batteries, and in relation to the methods and results of electrolysis, are 

 of such importance as to merit a whole address. The press also re- 

 cords numerous studies in actinometry, of the relations between chem- 

 ical composition and fluorescence and phosphorescence, as well as of 

 polychroism, and of the results of spectrum observations. Noteworthy 

 are the special applications of optical methods to the determination of 

 molecular structure, viz., the relations between chemical composition 



